Various methods for forming a multicolor image using electrophotography have been proposed. These methods are divided into the following two categories. One is to repeat a formation of an electrostatic latent image whose colors have been separated and the development thereof so as to superimpose the colors on a photoreceptor or transfer a toner image to a transfer material each time a development is carried out so as to superimpose colors on the transfer material. The other is to form a toner image in different colors on a plurality of photoreceptors provided correspondingly to the number of colors at the same time, and thereafter, to transfer them sequentially to a transfer material so as to obtain a multicolor image. The latter has an advantage that a color image is formed at a high. However, it requires a plurality of photoreceptors and light exposure means, thus causing a device of this kind to be complicated and large in its construction, which means that the device is expensive, i.e., it lacks practical use. In addition, both methods have a disadvantage that it is difficult to superimpose colors accurately. Accordingly, the registration of colors cannot be carried out satisfactorily.
In order to solve these problems, the inventors proposed a device in which a multicolor image is formed by carrying out an image exposure once on a photoreceptor before the present patent application was filed with the Japanese Patent Office. A multicolor formation is carried out by a device having a photoreceptor comprising a conductive substrate, a photoconductive layer, and an electrically insulating layer containing filter layers composed of plural kinds of finely divided filter units which serves as color separating means. By charging and exposing the photoreceptor, an image is formed on the photoreceptor according to the charge density generated at the boundary between the insulating layer and the photoconductive layer. Thereafter, by uniformly exposing the surface on which the image has been formed by means of a light which permeates through only one kind of filter units of the plural kinds of filter units, an electrical charged pattern is formed on the filter unit of the photoreceptor, and then, the electrical charged pattern is developed by a developing unit which accommodates toners in a specific color so as to form a monochrome toner image. After the electrification provided with photoreceptor is smoothed, the surface of the photoreceptor is uniformly exposed by a light which permeates through the filter units different from the filter unit previously charged and a development is carried out by a developing unit which accommodates toners in a color different from the above-described one, whereby a toner image different from the previously formed image is formed on the photoreceptor. Thereafter, necessary numbers of uniform image exposures and developments are carried out. As a result, toners in different colors adhere to the corresponding filter units in the photoreceptor to form a multicolor image. This method has been disclosed in Japanese Patent Publication Open to Public Inspection Nos. 225855/1985 and 65262/1986. (Hereinafter referred to as Japanese Patent O.P.I. Publication.) According to this multicolor image forming devices, it is unnecessary to carry out image exposures more than once. Therefore, a color doubling never occurs.
According to the above-described multicolor image forming method, a color reproduction is carried out by not superimposing colors at the same position, namely, by additive color process. Accordingly, the above-described color doubling does not occur and a color reproduction is performed favorably.
There is, however, a problem in the above method that, as shown in FIG. 3, since a respective toner adheres only to a corresponding filter, an image density thus obtained is unfavorable.